Chromium plating



. H SiF Patented July 21, 1931 UNITED STATES PATENT OFFICE ERW'IN SOHN AND RALPH J. PADDOGK, 0F IBELLEVUE, PENNSYLVANIA, ASSIGNORS T0 STANDARD SANITARY MFG. 00., OF PITTSBURGH, PEN N SYLVANIA, A CORPORA- TION OF NEW JERSEY GHROMIUM PLATING No Drawing.

This invention relates to chromium plating and more particularly to a process of electrodepositing chromium and to an electrolytic bath from which chromium can be satisfactorily deposited.

It is an object of this invention to provide an improved process and electrolytic bath for chromium plating.

It is a further important object of this invention to provide an electrolytic solution for chromium plating which has satisfactory throwing power and efliciency for commercial application and from which chromium can be electrodeposited as a bright andsmooth coating.

Other and further important objects of this invention will become apparent from the following description and appended claims.

It is generally known that in the chromic acidtype of chromium plating solutions, in addition to the chromic acid small quantities of an additional acid or acid radical are necessary to provide for satisfactory deposition. Of the acids or .acid radicals commonly used, sulphuric acid has probably found the widest application. The present invention concerns itself with a substitute either in part or in whole for the sulphuric acid.

We have now found that hydrofluosilicic acid, its alkali metal salts or, in general, any metallic salt of said'.acid which is dissolved by a chromic acid solution, can be used to replace sulphuric acid, either in whole or in part. Hydrofluosilicicacidisusuallyobtained as a solution containing from 28 to 35% The quantities hereinafter referred to are actual amounts of the acid (100%).

In carrying out our process, certain conditions are preferably standarized, as follows: Using leadanodes and a copper cathode, the temperature of the chromium plating bath is kept at 110 F. and a current density of 100 amperes persquare foot used with a voltage varying between 3.2 and 4 volts. It will, of course, be understood that these conditions can be varied depending upon the nature of the" work to be plated. However, with these conditions maintained, a plating bath having the following range of ingredi- Application filed November 5, 1928. Serial No. 317,475;

ents hasbeen found to give a chromium deposit with good cathode efliciencies:

CrO 150 to 450 gr. per liter H SiF 0.30 to 12.0 gr. per liter While a plating bath consisting of chromic acid and hydrofluosilicic acid alone will deposit chromium and with good cathode efliciency and throwing power as compared to a solution containing sulphuric acid, the nature of the chromium deposit is not entirely plating bath is as follows:

CrO 208 gr. per liter H SO 0.35 gr. per liter H SiF 0.60 gr. per liter Using our preferred plating solution, a

satisfactory cathode efliciency, say of 16%,

can be obtained and the chromium plated out is bright and smooth. Furthermore, the throwing power of the solution is very good. Further additions of hydrofluosilici'c acid do not greatly affect the efficiency until about 2.6 grams per liter of hydrofluosilicic acid are present in the solution, but the throwing power of the solution decreases with the first addition, at which point 0.60 grams per liter of hydrofluosilicic acid had been added. A. solution containing 0.35 grams per liter of sulphuric acid and 0.60 grams per liter of hydrofluosilicic acid is a good working solution, having good throwing power and efficiency.

The fact that the sulphate ioncontent of the chromium plating bath can be reduced below thewquivalent of 1 gr. per liter of H 80 by replacing a part of the sulphuric acid normally used with,hydrofluosilicic acid In place of hydrofluosilicic acid, alkali mo metal salts of the acid may be used, such as sodium fluosilicate, or as stated, most any metallic salt which is soluble in a chromic acid solution, for instance, chromium fiuosilicate. For calculating the amount of metallic fiuosilicate, the equivalent of hydrofluosilicic acid is used within the above range. Like wise, in place of sulphuric acid, introduced as such, metallic salts of sulphuric acid may be used. It will therefore be understood that by the terms sulphuric acid and hydrofiuosilicic acid, as used in the claims, is meant either the free acids or salts of said acids capable of furnishing in solution the respective acid radicals.

Due to the fact that commercial chromic acid, in general, contains appreciable quantities of sulphates, it is susually necessary to take special steps to control the sulphate content of a plating solution prepared from commerical chromic acid and hydrofluosilicic acid. A method of control has been described in the copending application of Erwin Sohn, Serial No. 294,940, filed July 23, 1928, which comprises determining the sulphate content of the solution analytically and adding a' calculated amount .of barium carbonate to. precipitate the excess of sulphate ions as barium sulphate. By this method the sulphate content of the solution may be brought down to 0.35 grams per liter of equivalent ILSO. or less. If the sulphate content is brought below the desired amount, sufiicient sulphuric acid may be added and then the desired quantity of hydrofluosilicic acid introduced until the solution possesses the desired efiiciency and throwing power. It

has been found advisable to filter the solution after the precipitation of excess sulphate. with barium carbonate.

In contrast with other plating solutions, it has been found that boric acid has no value in a chromium plating solution containing chromic acid and hydrofluosilicic acid. In fact, the efiect of boric acid is harmful.

We are aware that numerous details of the process may be varied through a wide range without departing from the principles of this invention, and we, therefore, do not purpose limiting the patent granted hereon otherwise than necessitated by the prior art.

, We claim as our invention:

1. The process of chromium plating, which comprises passing an electric current through a bath containing from 150 to 450 grams of chromic acid, from 0.35 to 1.05 grams of sulphuric acid and from 0.30 to 3.0 ams per liter of hydrofluosilicic acid to the article to be plated as cathode.

2. The process of chromium plating, which comprises passing an electric current through a bath containing approximately 200 grams of chromic acid, approximately 0.35 grams of sulphuric acid, and approximately 0.60 grams of hydrofluosilicic acid per liter of solution to the article to be plated as cathode.

3. A chromium plating bath, consisting of a solution of 150 to 450 grams of chromic acid, 0.35 to 1.05 grams of sulphuric acid and 0.30 to 3.00 grams of hydrofluosilicic acid. 1

4. A chromium plating bath, consisting of approximately 200 grams of chromic acid,

0.35 grams of sulphuric acid and 0.60 grams of hydrofluosilicic acid per liter of solution. 5. A chromium plating bath of high throwing power, comprising a 15 to 45% solution of chromic acid having a mixed stable acid content of from 0.65 to 4.05 grams per liter of solution, said stable acid content being composed of a relatively strong and a relatively weak inorganic acid, said relatively weak inorganic acid being hydrofluosilicic acid in an amount ranging from 0.6 to 3.0'grams per liter of solution.

In testimony whereof, we have hereunto subscribed our names at Pittsburgh, county of Allegheny, Penna.

ERWIN SOHN. RALPH J. PADDOCK. 

